1. Field of the Invention
The invention relates to the art of chlorine dioxide production, and in particular, to the continuous production of chlorine dioxide in a plug flow reactor by reduction of an alkali metal chlorate or chloric acid with a reducing agent. In a preferred embodiment, the process of the invention uses hydrogen peroxide as the reducing agent.
2. Description of the Prior Art
Chlorine dioxide used in aqueous solution is of considerable commercial interest, mainly in pulp bleaching, but also in water purification, fat bleaching, removal of phenols from industrial wastes, etc. It is therefore desirable to provide processes in which chlorine dioxide can be efficiently produced.
In existing processes for the production of chlorine dioxide, chlorine gas is often formed as a by-product, due to the use of chloride ions as a reducing agent. The basic chemical reaction involved in such processes is: EQU ClO.sub.3.sup.- +Cl.sup.- +2H.sup.+ .fwdarw.ClO.sub.2 +1/2Cl.sub.2 +H.sub.2 O [1]
The chlorate ions are provided by alkali metal chlorate, preferably sodium chlorate, the chloride ions by alkali metal chloride, preferably sodium chloride, or by hydrogen chloride, and the hydrogen ions are provided by mineral acids, generally sulfuric acid and/or hydrochloric acid.
In the production of chlorine dioxide with chloride ions as the reducing agent according to reaction [1], half a mole of chlorine is produced for each mole of chlorine dioxide. This chlorine gas by-product has previously been used as such in paper mills as a bleaching agent in aqueous solution. However, increased environmental demands have resulted in a change-over to pure chlorine dioxide bleaching. To achieve pure chlorine dioxide bleaching there is an increasing demand for chlorine dioxide manufacturing processes which do not produce chlorine as a by-product.
One known way of reducing the chlorine by-product is to use reducing agents which do not produce chlorine as a by-product. One example is in the so-called "Solvay" process, wherein alkali metal chlorate is reduced in an acid medium with methanol as the reducing agent. Another example is in the "Mathieson" process, in which chlorate is reduced with sulfur dioxide in a sulfuric acid-containing medium. These processes use methanol and sulfur dioxide, respectively, as indirect reducing agents, and hence the rate of reaction is very slow. In U.S. Pat. No. 4,081,520, an allegedly more effective "Solvay" process is described using a reduced pressure and a high acid normality in a single vessel reactor.
The direct reducing agent in the case of methanol and sulfuric acid reactions is chloride ion reacting according to reaction [1]. The chlorine produced then reacts with methanol to regenerate chloride ions according to the reaction: EQU CH.sub.3 OH+3Cl.sub.2 +H.sub.2 O.fwdarw.6Cl.sup.- +CO.sub.2 +6H.sup.+[ 2]
or with sulfur dioxide according to the reaction: EQU Cl.sub.2 +SO.sub.2 +2H.sub.2 O.fwdarw.2HCl+H.sub.2 SO.sub.4[ 3]
According to one prevalent theory holding that chloride ion must be present, it is often necessary to continuously add a small amount of chloride ion in order to obtain a steady production. Due to the continued presence of chloride ion, even with methanol or sulfur dioxide as the reducing agent, a certain amount of chlorine by-product is produced. According to U.S. Pat. No. 4,081,520, operating with methanol as reducing agent, the amount of chlorine by-product produced is decreased with increasing acid normality in the reaction medium. The reaction rate is also increased with increasing acid strength. At a low acid normality, the reaction is so slow that it is of no commercial interest. However, the drawback with a high acid strength in the reaction medium is, in addition to more corrosion in the equipment, the production of an acid salt in the form of sesquisulfate (Na.sub.3 H(SO.sub.4).sub.2) or bisulfate (NaHSO.sub.4). This occurs at an acid normality of from above about 5N to about 12N. An acid salt results in loss of acid in production and costs for neutralization of the salt. From about 2 N to about 5N acid normality, a neutral alkali metal salt (alkali metal sulfate) is formed.
It is also known to speed up the reaction rate at low acidities by using catalysts both with chloride and methanol as the reducing agent. U.S. Pat. No. 3,563,702 discloses catalysts for chloride reduction. However, catalysts are expensive and thus increase the production costs.
Another drawback with methanol as the reducing agent is the possible formation of chlorinated organic compounds, from by-products of methanol, in the downstream bleaching process. It is well known that the efficiency of the added methanol is lowered due to side reactions wherein formaldehyde and formic acid are formed. Also, some of the methanol leaves the reactor without having participated in the reduction reaction. The corresponding ether and ester are probably there as well. It could be expected that reactions can occur in the bleaching train with aldehyde, acid, ether and ester, resulting in chlorinated organic compounds.
In U.S. Pat. Nos. 5,091,166 and 5,091,167, the draw-backs of using methanol as a reducing agent are addressed by substituting hydrogen peroxide for methanol. These patents disclose production of chlorine dioxide using a single vessel process under subatmospheric pressure. Alkali metal chlorate is reduced with hydrogen peroxide as the reducing agent in an aqueous reaction medium containing sulfuric acid. The reaction medium is maintained at its boiling point of between 50.degree. C. and 100.degree. C. such that water is evaporated therefrom, forming steam. A gaseous mixture containing the steam, produced chloride dioxide, and by-product oxygen is withdrawn from the vessel.
In the reaction medium an alkali metal salt crystallizes and is removed. The type of salt crystallized is a function of the acid normality of the reaction medium. At an acid normality of between 2 and 4, a neutral sodium sulfate salt, for example, Na.sub.2 SO.sub.4, forms. At higher acid normalities, a sesquisulfate salt or a bisulfate is formed.
While the processes disclosed in U.S. Pat. Nos. 5,091,166 and 5,091,167 are a great improvement over the prior art methanol processes, they are performed in a single vessel process (SVP.RTM.) reactor in which the generation and separation of chlorine dioxide are carried out in a single reaction vessel maintained at the boiling point of the reaction medium. Kinetically, the single reaction vessel functions as a constant flow stirred tank reactor ("CFSTR" or "CSTR"). There are indeed numerous advantages to this type of reaction vessel. By maintaining the reaction medium at its boiling point, the evolved chlorine dioxide is diluted with steam, thereby reducing explosion risk. Alkali metal salt concentration in the reaction medium is maintained at saturation, resulting in the alkali metal salt being continuously deposited and easily removed.
On the other hand, single vessel processes require long residence times to obtain an acceptable rate of conversion. Long residence times require either a low flow rate through the reaction vessel (and subsequent low production rate) or a large vessel size. Production requirements, at least for large consumers of chlorine dioxide, dictate that residence times be maintained via a large reaction vessel. Paper pulp mills, for example, require between 15 and 60 tons per day (TPD) of chlorine dioxide, and this production level requires large scale equipment.
A typical SVP.RTM. reaction vessel for 40 TPD of chlorine dioxide is about 10 feet in diameter and has a volume of about 8800 gallons. Since chlorine dioxide is produced on-site (for safety reasons), the SVP.RTM. reaction vessel and related process equipment must be shipped to and installed at the location of use. Shipping costs are high due to the weight and bulk involved. Also, the size of the reaction vessel requires the commitment of considerable plant space.
In addition to the costs and the space requirements of an initial installation, further costs are incurred if an upgrade in chlorine dioxide production capacity using SVP.RTM. technology is required. Such an upgrade would require removal of the existing generator vessel and replacement with an even larger vessel, essentially duplicating the initial installation costs. Alternatively, the upgrade would require the addition of a secondary SVP.RTM. generator having additional installation costs and space requirements. In either case, the upgrade would be expensive.
The concept of plug flow reactors has heretofore been applied to various chemical processes and offers the advantage of small size with reasonable production rates. However, plug flow reactors were not believed feasible for producing chlorine dioxide, due to the relatively slow kinetics of uncatalyzed reaction schemes. Catalyzed systems were also deemed unsuitable for plug flow processes due to accumulation and clogging of the equipment by the solid phase catalyst.
In very small scale processes, non-CSTR, continuous chlorine dioxide reactions have been used successfully. For example, in U.S. Pat. No. 5,061,471, there is disclosed a process for continuous production of chlorine dioxide using alkali metal chlorate, sulfuric acid and sulfur dioxide as the reducing agent. This process is suitable for small scale chlorine dioxide applications such as treatment of drinking water, etc. This patent does not teach plug flow, however, since the concentration profile in the reactor is uniform, which approximates a CSTR.
There is accordingly a need in the art for a chlorine dioxide process which has the advantages of low chlorine by-product generation and high production rate and which also has reduced installation and upgrading costs compared to processes using single vessel process generators.